A battery capacity calculation method for calculating battery capacity is known. In the battery capacity calculation method, a current integrated charge-rate change amount is calculated based on an integration value of sensor current for a predetermined current-integration period within a charging period of a battery, open circuit voltage at the start and end of the predetermined current-integration period is estimated based on the state quantity of the battery, charge rates at the start and end of the predetermined current-integration period is calculated based on the estimated open circuit voltage, and an open circuit voltage charge-rate change amount is calculated based on their difference. Then, a capacity ratio, which is a ratio of the current integrated charge-rate change amount to the open circuit voltage charge-rate change amount is calculated and a battery capacity of the battery is calculated by multiplying the capacity ratio to the initial battery capacity of the battery (Japanese Patent Application JP 2011-215125 A).
However, there has been a problem that calculation accuracy of the capacity ratio is low due to detection errors of a sensor.